She said: “I’m not going to support Boris Johnson or Jeremy Corbyn, They are not up to the job.”

And she said she wanted the party to win more than 300 seats in an election.

To secure a majority in the House of Commons, a party has to win more seats than all the other parties put together. At the moment, that means winning at least 326 seats – more to ensure a comfortable majority.

She dismissed the view that the Liberal Democrats were unlikely to win more than 300 seats.

“I reject this suggestion that you go into a general election campaign, particularly in these volatile political times and somehow people have to accept they don’t have a genuine choice.

“They do not have to choose between Boris Johnson and Jeremy Corbyn because frankly that choice is not good enough. Neither of those men is fit to be our prime minister.”

Asked if she would act as “kingmaker” for the Conservatives or Labour in the event of a hung Parliament, Ms Swinson said: “I’m not going to support Boris Johnson or Jeremy Corbyn to be prime minister.”

On Sunday, the Liberal Democrats backed a policy of campaigning for Remain if there is a general election.

She said: “As a party that wholeheartedly believes that our best future is within the European Union, we need to give the British people the chance to vote for that by saying that if you elect a Liberal Democrat government we will stop Brexit by revoking Article 50.”